Color cathode ray tube of the pluralbeam,single electron gun type



Dec. 15, 1970 SUgU YOSHlDA ET AL 3,548,249

COLOR CATHODE RAY TUBE OF THE PLURAL-BEAM, SINGLE ELECTRON GUN TYPEFiled Feb. 5, 1969 2 Sheets-Sheet 1 0-500M 0' -400 l4 0- 400M BINVENTORS SUSUMU YOSHIDA AKIO OHGOSHI SENR! MIYAOKA MINORU MORIOATTORNEY D66. 15, 1970 SUSUMU YOSHIDA ET AL 3,548,249

COLOR CATHODE RAY TUBE OF THE PLURAL-BEAM, SINGLE ELECTRON GUN TYPE 2Sheets-Sheet 2 Filed Feb. 5, 1969 INVENTORS SUSUMU YOSHIDA AKIO OHGOSHISENRI MIYAOKA MINORU MORIO Ra a-47.

ATTORNEY United States Patent U.S. Cl. 315-13 7 Claims ABSTRACT OF THEDISCLOSURE A color cathode ray tube preferably of the single gun, pluralbeam type wherein the beams originate on a horizontal or verticalstraight line and are directed into a field produced by ahorizontal-vertical electromagnetic deflection yoke at predeterminedincident angles to each other so as to converge at a color screen, isprovided with a magnetic yoke, for example, secured to the electron beamconvergence plates at the end of the electron gun so as to be disposedin the path of the leakage flux pro duced by the horizontal-verticalelectromagnetic deflection yoke, and thereby impart a further verticalor horizontal deflection field to at least one of the plural beams forcorrecting a deviation between the positions of rasters on the colorscreen produced by the plural beams.

This invention relates generally to color picture tubes of thesingle-gun, plural-beam type, and particularly to tubes of that type inwhich the plural beams are passed through the optical center of a commonelectron lens by which the beams are focussed on the color phosphorscreen.

In single-gun, plural beam color picture tubes of the described type,for example, as specifically disclosed in the copending US. application,Ser. No. 607,414, filed Jan. 12, 1968, now Pat. No. 3,448,316, andhaving a common assignee herewith,- three laterally spaced electronbeams are emitted or originated by a beam generating or cathode assemblyand directed in a common substantially horizontal or vertical plane withthe central beam coinciding with the optical axis of the single electronlens and the two outer beams being converged to cross the central beamat the optical center of the lens and thus emerge from the latter alongpaths that are divergent from the optical axis. Arranged along suchdivergent paths are pairs of convergence deflecting plates havingvoltages applied thereacross to deflect the divergent beamssubstantially in the plane of origination thereof for causing all beamsto converge at a point on the apertured beam selecting grid or shadowmask associated with the color screen. After passing between theconvergence deflecting plates, the beams are acted upon by the magneticfields resulting from the application of horizontal and vertical sweepsignals to the corresponding coils of a deflection yoke, whereby thebeams are made to scan the screen in the desired raster. It will beapparent that, when the three beams are decected by the yoke from apoint of convergence at the center of the screen, as during scanning,the distances that such beams travel through the magnetic fields of thedeflection yoke are relatively varied and spherical aberration results,that is, the beams undergo different degrees of deflection resulting inmisconvergence of the beams, particularly when the latter are directedat corner portions of the screen.

Although certain aspects of the above described misconvergence can becorrected by suitably shaping and di- 3,548,249 Patented Dec. 15, 1970mensioning the horizontal and vertical deflection coils, for example, ashereinafter described in detail, there remains a deviation of the rasterin the central beam with respect to the rasters of the other two beams,particularly at the top and bottom of the screen in the case of thebeams originating in a horizontal plane.

Accordingly, it is an object of this invention to avoid the abovementioned deviation of the rasters from each other, particularly at thetop and bottom of the screen of a color picture tube of the describedtype, without resorting to complex dynamic convergence devices for thatpurpose.

Another object is to achieve the desired registration of the rasters bysubjecting one or more of the electron beams to a correction field whichis not applied to the remainder of the electron beams.

A further object is to produce the correction field for achievingregistration of the rasters by collecting leakage flux from thedeflection yoke and concentrating the collected flux in a space throughwhich one, for example, the central, electron beam passes, whereby toimpart an additional deflection to the central beam.

In accordance with an aspect of this invention, a singlegun, plural-beamcolor picture tube, as described, is provided with magnetic yoke membersdisposed at opposite sides of the path of the central beam, preferablyat the entrance for the latter between the convergence deflectingplates, and each of the yoke members has a straight portion extending atright angles to the plane in which the beams originate and beingdisposed between the central beam and the adjacent side beam, with endportions provided on the ends of the straight portion and extending atsubstantial angles to the latter in directions away from the centralbeam to act as pole pieces for collecting leakage flux from thedeflection yoke coil intended to deflect the beams at right angles tosaid plane, with the collected flux being concentrated in the spacebetween the straight portions of the yoke members and hence acting onthe central beam passing through such space.

The above, and other objects, features and advantages of this invention,will become apparent from the following detailed description of anillustrative embodiment which is to be read in conjunction with theaccompanying drawings, in which:

FIG. 1 is a schematic horizontal cross-sectional view showing aplural-beam, single-electron gun type color cathode ray tube of the typeto which the present invention can be applied;

FIGS. 2A and 2B are front and side views showing the mechanicalarrangement of deflection yoke means applicable to the color cathode raytube of FIG. 1;

FIG. 3 is a diagrammatic view illustrating the manner in which the beamsare deflected and converged;

FIG. 4 is a diagrammatic view illustrating the manner in which rastersare formed and the deviations thereof to be corrected by this invention;

FIG. 5 is a horizontal cross-sectional view of the main portion of thecolor tube of FIG.l, but shown with an embodiment of the presentinvention applied thereto;

FIG. 6 is a transverse sectional view taken along line XX of the tube ofFIG. 5 and FIG. 7 is a diagrammatic view showing the magnetic fielddistribution occurring in the embodiment of the invention illustrated byFIGS. 5 and 6.

In order to provide a better understanding of the present invention, athree-beam single-gun type color cathode ray tube of the type to whichthis invention may be applied is described below.

Referring to the drawings, and initially to FIG. 1 thereof, it will beseen that the single-gun plural-beam color picture tube 10 there shownmay comprise a glass envelope (shown in dotted line) having a neck, anda cone extending from the neck to a color screen S provided with theusual arrays of color phosphors S S and S and with an aperture beamselecting grid or shadow mask G Disposed within the neck is a singleelectron gun having cathodes K K and K each of which is constituted by abeam-generating source with the respective beam-generating surfacesthereof disposed as shown in a plane which is substantiallyperpendicular to the axis of the electron gun. In the embodiment shown,the beam-generating surfaces are arranged in a straight line so that therespective beams B B and B emitted therefrom are directed in asubstantially horizontal plane containing the axis of the gun, with thecentral beam B being coincident with such axis. A first grid G is spacedfrom the beam-generating surfaces of cathodes K K and K and hasapertures g g and g formed therein in alignment with the respectivecathode beamgenerating surfaces. A common grid G is spaced from thefirst gird G and has apertures g g and g formed therein in alignmentwith the respective apertures of the first grid G Successively arrangedin the axial direction away from the common grid G are open-ended,tubular grids or electrodes G G and G respectively, with cathodes K Kand K grids G and G and electrodes G G and G being maintained in thedepicted assembled positions thereof, by suitable, non-illustratedsupport means of an insulating material.

For operation of the electron gun of FIG. 1, appropriate voltages areapplied to the grids G and G and to the electrodes G G and G Thus, forexample, a voltage of to minus 400 v. is applied to the grid G a voltageof 0 to 500 v. is applied to to the grid G a voltage of 13 to 20 kv. isapplied to the electrodes G and G and a voltage of O to 400 v. isapplied to the electrode 6,, with all of these voltages being based uponthe cathode voltage as a reference. As a result, the voltagedistributions between the respective electrodes and cathodes, and therespective lengths and diameters thereof, may be substantially identicalwith those of a unipotential-single beam type electron gun which isconstituted by a single cathode and first and second, single-aperturegrids.

With the applied voltage distribution as described hereinabove, anelectron lens field will be established between grid G and the electrodeG to form an auxiliary lens L as indicated in dashed lines, and anelectron lens field will be established around the axis of the electrode6,, by the electrodes G G and G to form a main lens L, again asindicated in dashed lines. In a typical use of electron gun A, biasvoltages of 100 v., 0 v., 300 v., 30 kv., 200 v., and v. may be appliedrespectively to the cathodes K K and K the first and second grids G andG and the electrodes G G and G Further included in the electron gun ofFIG. 1 are electron beam convergence deflecting means F which compriseshielding plates P and P disposed in the depicted spaced, relationshipat opposite sides of the gun axis, and axially extending, deflectorplates Q and Q which are disposed, as shown, in outwardly spaced,opposed relationship to shielding plates P and P, respectively. Althoughdepicted as substantially straight, it is to be understood that thedeflector plates Q and Q may, alternatively, be somewhat curved oroutwardly bowed, as is well known in the art.

The shielding plates P and P are equally charged and disposed so thatthe central electron beam B will pass substantially undeflected betweenthe shielding plates P and P, while the deflector plates Q and Q havenegative charges with respect to the plates P and P so that respectiveelectron beams B and B will be convergently deflected as shown by therespective passages thereof between the plates P and Q and the plates Pand Q. More specifically, a voltage V which is equal to the voltageapplied to the electrode G may be applied to both shielding plates P andP, and a voltage V which is some 200 to 300 v. lower than the voltage Vis applied to the respective deflector plates Q and Q so that respectiveshielding plates P and P are at the same potential, and a deflectingvoltage difference, or convergence deflecting voltage is providedbetween the respective plates P and Q and P and Q. It is thisconvergence deflecting voltage V which will impart the requisiteconvergent deflection to the respective electron beams B and B D is astatic convergence voltage generating circuit which is applied betweenthe electrode plates P and Q, and between the electrode plates P and Q.

'In operation, the respective electron beams B B and B which emanatefrom the beam generating surfaces of the cathodes K K and K will passthrough the respective grid apertures g g and g to be intensitymodulated with what may be termed red, green, and blue intensitymodulation signals applied between the cathodes and the first grid G Therespective electron beams will then pass through the common auxiliarylens L to cross each other at the center of the main lens L and emergefrom the latter with beams B and B diverging from beam B Thereafter, thecentral electron beam B will pass substantially undeflected betweenshielding plates P and P since the latter are at the same potential.When electron beam B passes between plates P and Q and electron beam Bpasses between plates P and Q, they will converge as a result of theconvergence deflecting voltage applied therebetween. The system of FIG.1 is so arranged that the electron beams B B and B will desirablyconverge or cross each other at a common spot centered in an aperturebetween adjacent grid wires g of the beam selecting grid or mask G so asto diverge therefrom to strike the respective color phosphors of acorresponding array on screen S. More specifically, it may be noted thatthe color phosphor screen S is composed of a large plurality of sets orarrays of vertically extending red, green, and blue phosphor stripes ordots S S and S with each of the arrays or sets of color phosphorsforming a color picture element as in a chromatron type color picturetube. Thus, it will be understood that a common spot of beam convergencewill correspond to one of the color picture elements.

The voltage V may also be applied to the lens electrodes G and G and tothe screen S as an anode voltage in a conventional manner through anon-illustrated graphite layer which is provided on the inner surface ofthe cone of the tube envelope. The grid wires of screen grid G may havea post-focussing voltage ranging, for example, from 6 to 7 kv. appliedthereto. Thus, to summarize the operation of the depicted color picturetube of FIG. 1, the respective electron beams B B and B will beconverged at screen grid G and will diverge therefrom in such mannerthat electron beam B will strike the blue phosphor S electron beam Bwill strike the green phosphor S and electron beam B will strike the redphosphor S of the array or set corresponding to the grid aperture atwhich the beams converge.

Electron beam scanning of the face of the color phosphor screen iselfected in a conventional manner, for example, by horizontal andvertical electromagnetic deflection means 20 indicated in broken lines.Deflection means 20 may be constructed as a deflection yoke means havinghorizontal and vertical deflection coils wound in a saddlelike ortoroidal form. By selecting the winding angle and position of thedeflection coils so that a pin-cushion shaped horizontal deflectionfield is produced by the horizontal deflection coil and barrel shapedvertical deflection field is produced by the vertical deflection coil,it is possible to eifect dynamic convergence by the magnetic fieldproduced by the deflection yoke means, as described in detail in USpatent application Ser. No. 753,694, filed Aug. 19, 1968, now Pat. No.3,500,114, and having a common assignee herewith.

An example of a deflection yoke means capable of producing a pin-cushionshaped deflection field, and a barrel shaped vertical deflection fieldis shown at 20 in FIGS.

2A and 2B. In FIGS. 2A and 2B, a deflection yoke 21 is provided on ayoke supporting annular member 24 which is enlarged at its front end inthe form of a funnel. A pair of vertical deflection coils V.,, and V aresymmetrically wound in a toroidal form on deflection yoke 21 withrespect to horizontal plane HH passing through the axis of the yoke.These vertical deflection coils are connected, for example, in serieswith each other. In order to produce a barrel shaped vertical deflectionfield, the winding angle 0 at which the vertical deflection coils V andV are wound on the yoke 21, is selected to be greater than that whichwould produce a rectangular vertical deflection field. In the case of auniformly distributed winding, the winding angle is set between 120 and160.

A pair of saddle-shaped horizontal deflection coils H and H, extendwithin annular member 24 and are symmetrically located with respect tohorizontal plane HH. These horizontal deflection coils are connected,for example, in series with each other. In order to produce apin-cushion shaped field, the left hand side effective coil portion 22Lof horizontal deflection coil H and the left hand side eflective coilportion 23L of horizontal deflection coil H, are disposed in contact, orin closely spaced relationship with each other. Similarly, the righthand side effective coil portion 22R of coil H and the right hand sideeffective coil portion 23R of coil H, are disposed in contact, orclosely spaced relationship with each other. The winding angles 0;; ofcoil portions 22L and 23L, and of coil portions 22R and 23R are selectedto be between 120 and 130. The front portions of coils H and H adjacentthe wide end of support 24 are constructed in the form of a Windingrepresented by the nth power of the cosine, or cos where n is a positivenumber between 2 and 7. The rear portions of coils H and H areconstructed in the form of a winding represented by the mth power of thecosine, or cos where m is a positive number between 1 and 3.

With the above arrangement, even if horizontal and vertical convergencevoltage and current generating circuits, and vertical convergence meansare omitted, dynamic convergence in both the horizontal and verticaldirections can be effected with respect to the three beams B B and Bwhen these beams are made to scan screen S.

The three beams B B and B when being deflected horizontally andvertically are located in a common plane which is inclined with respectto the horizontal plane HH through an angle corresponding substantiallyto the angle of vertical deflection, as the beams are always arranged ona substantially horizontal line. However, the three beams in the commonplane enter into the deflection yoke means 20 at difference incidentangles due to convergence means F. Thus, if the deflection yoke means ofFIGS. 2A and 2B is not employed, there is a tendency that the threebeams will cross each other at a position which is short of screen Swhen these beams are directed to the left or right hand side portionthereof. Thus, as diagrammatically shown in FIG. 3, beams B and B whichare at the left and right of central beam B at the deflection positionshown, tend to land on screen S to the right and left, respectively, ofbeam B when these beams are directed to the left hand side portion ofthe screen. However by using the deflection yoke means of FIGS. 2A and2B, beam B is deflected from its deflection center position D across aminimum field position corresponding to deflection center position D andB is deflected from its deflection center position D through arelatively strong portion of the pin-cushion type field. Thus. the threebeams can be made up to converge accurately with each other at thescreen as shown by dotted lines in the drawing. For verticaldeflections, if the de flection yoke means of FIGS. 2A and 2B is notused, the three beams tend to cross each other short of the screen atthe opposite sides as in the horizontal deflection. By

using the deflection yoke means of FIGS. 2A and 2B, however, the threebeams are subjected to substantially the same component of a barrel typefield so as to converge with each other at screen S, since they are notvertically spaced apart from each other.

Thus by winding the horizontal deflection coil in a saddle-like form,and by winding the vertical deflection coil in a toroidal formcorresponding with the curved surface of screen S, vertical dynamicconvergence can be effected without using any vertical dynamicconvergence voltage and current generating circuits. The configurationsof the pin-cushion and barrel magnetic fields can be determined bywinding angle 0 of vertical deflection coils V and V and their positionson yoke 21, and winding angle 0 of horizontal deflection coils H and H,and their positions within support 24. Thus, effective convergence canbe achieved without providing dynamic convergence means as normallyrequired by convention color cathode ray tubes, or on the other hand,more eflective convergence can be obtained by using such dynamicconvergence means at the same time.

The horizontal deflection coils in saddle-like form can be produced aseasily as if they were wound in toroidal form. Furthermore, due to thefact that the horizontal deflection frequency is sufliciently higherthan the vertical deflection frequency, power consumption for thedeflection may be reduced to 60% of that required by horizontaldeflection coils wound in toroidal form. Likewise, the power consumptionof the vertical deflection coils would be higher if they were wound in asaddle-like form than in a toroidal form. Therefore, the horizontaldeflection coils should be wound in a saddle-like form,and the verticaldeflection coils in a toroidal form. Moreover, by winding the horizontaldeflection coils in a saddle-like form, it is possible to easily changethe configurations of the portion of the horizontal deflection field onthe screen side and that on the electron gun side so that, for example,one of the field portions can be of the barrel type, while the otherfield portion is of the pin-cushion type. while the remainder of thehorizontal deflection field is either a pin-cushion type or a barreltype field. This would become diflicult to achieve if the horizontaldeflection coils were wound in toroidal form.

Usually rasters appearing on the screen tend to be sub jected topin-cushion distortion due to the configuration of the screen in spiteof setting the focussing adjustment to achieve the best possible beamfocussing. However, by making the horizontal deflection field portion atthe screen side to be pin-cushion shaped, and the horizontal deflectionfield on the electron gun side to be barrel shaped, in accordance withthe saddle-like configuration of the horizontal deflection coils, it ispossible to easily correct pincushion distortion of the rastersresulting from curvature of the screen.

With the foregoing arrangement, however, the following undesirableeffect is produced due to the fact that the vertical deflection field isof the barrel type configuration. When the three beams B B and Boriginating in a common horizontal plane are vertically deflected toscan screen S at a high or low position, rasters L L and L resultingfrom the red, green, and blue beams B B and B should be located on acommon horizontal line U as indicated in FIG. 4. However, it has beenfound that when rasters L and L resulting from side beams B and B are inregistration with each other on the midpoint of horizontal line U,raster L extending through the midpoint, tends to be slightly lowered onthe right hand side. Raster L also tends to be lowered at the left handside, and raster L resulting from center beam B tends to be shiftedcloser to the center of screen S than rasters L and L It may be assumedthat such tendency is mainly due to the fact that the verticaldeflection field is barrel shaped as described above. If the verticalsweep voltage is given a suitable waveform, the described inclination ofrasters L and L can be substantially eliminated or at least madenegligible. However. it is relatively diflicult, by such means, toeliminate the deviation between the positions of rasters L and L and theposition of raster L In accordance with the present invention, there isprovided a simple arrangement by which, even if a deviation which cannotbe neglected occurs between the positions of rasters L and L and thepositions of raster L such a deviation can be easily eliminated. Morespecifically, the foregoing elimination of the deviation between rasterL and rasters L and L at the top and bottom of screen S is achieved byrelatively increasing the magnetic field effect acting on center beam Bas compared with the magnetic field effect acting on beams B and B Therelatively increased magnetic field effect is attained, for example, byproviding a correcting magnetic field acting in the same direction asthe vertical deflection field, in the case where the beams originate ina horizontal plane, and through which only the central beam B is made topass. When the invention is applied to a color cathode ray tube of thetype shown on FIG. 1, the correcting magnetic field may beadvantageously disposed at the entry for beam B to the convergencedeflecting means F.

More specifically, as shown on FIGS. and 6, such convergence deflectingmeans F may have its electrode plates P and P attached to the endsurface of cylindrical grid G through conductive angle members 51 and51, respectively. Electrode plates Q and Q are attached to insulatingmembers 53 and 53 mounted on support pins 52 and 52 extending from theelectrode plates P and P respectively. Further, a brush or coil springmember 55 is secured to a bracing member 54 bridging the free ends ofelectrode plates P and P so as to maintain a spacing between theseelectrode plates. Member 55 is in electrical contact with a conductivelayer 56 extending over the inner surface of the neck portion N, and towhich an anode voltage V is applied by way of an anode button (notshown). Hence, such anode voltage is applied to electrode plates P andP. Plates Q and Q are connected with each other through a conductor wire57, and a conductor Wire 59 extends from electrode plate Q for exampleto a button 58 provided in the neck portion N for example, so that avoltage that is 200 to 300 v. lower than anode voltage V can be therebyapplied to electrode plates Q and Q.

In accordance with this invention, the magnetic correcting field isprovided by mounting magnetic yoke members Y and Y on the outer surfacesof electrode plates P and P, respectively, adjacent angle members 51 and51. Each of these magnetic yoke members Y and Y may include a flat orstraight portion 60 extending across the corresponding electrode plate Por P, and bent end portions 61 and 62 which extend outwardly from theopposite ends of straight portion 60.

With such an arrangement, magnetic leakage flux from the verticaldeflection field produced by deflection yoke means 20 can pass throughthe opposing magnetic yoke members Y and Y, as indicated by the arrows63 on FIG. 5. It is obvious that because of the described configurationof magnetic yoke members Y and Y, the field distribution density in thespace between straight portions 60, and through which center beam Bpasses is higher than the field distribution densities in the portionsthrough which side beams F and F pass, as shown in FIG. 6. If it isassumed that the vertical deflection magnetic flux enters at magneticyoke member Y, it will be apparent that the leakage flux occurringoutside magnetic yoke member Y will be collected thereby and themagnetic flux thus collected will arrive at magnetic yoke member Y, andthen be expanded.

Where each of magnetic yoke members Y and Y is provided with the bentportions 61 and 62, the magnetic flux densities between such bentportions 61 and 62 are lower than the magnetic flux density betweenstraight portions 60 thereof. In this way, the vertical deflection ofcenter beam B is increased more than the vertical deflections of sidebeams B and B Thus, it is possible to eliminate deviations of theposition of center beam B from the positions of side beams B and BConsequently, rasters L L and L resulting from beams B B and B canappear substantially along the common horizontal line U.

The above described corrective effect can be produced merely byproviding magnetic yoke members Y and Y at the entry to convergencemeans F. Furthermore, the magnetic correcting field needed for assistingthe vertical deflection of the center beam can be obtained from theleakage component of the vertical deflection field produced bydeflection yoke means 20. Therefore, there is no need to provide anyspecial electromagnetic means to produce the deflection correctingfield.

Although the leakage component of the vertical deflection field is usedfor aiding the vertical deflection of the center beam in the describedembodiment, it will be apparent that it is also possible to produce theaforementioned effect by providing additional, external electromagnetmeans or permanent magnet means to produce magnetic flux in yoke membersY and Y. Such additional external magnet means would be secured alongneck portion N and arranged to produce the pattern of field densitiesacross yoke members Y and Y as shown in FIG. 7.

In the above description of the invention, it has been assumed that theyoke members Y and Y according thereto are employed to correct adeviation of the raster of center beam B from the rasters of side beamsB and B that may remain even when the horizontal and vertical deflectioncoils are given the configuration described with reference to FIGS. 2Aand 2B. However, it will be appreciated that, when the horizontal andvertical deflection coils are not given the configuration of FIGS. 2Aand 2B, the yoke members Y and Y' can be still employed to correct theaforementioned deviation between the rasters of the three electronbeams.

Further, the application of the present invention is not limited to thetube arrangement of FIG. 1, wherein plural beams are made to cross eachother at the center of a main focussing lens of a single electron gunand subsequently pass through convergence deflecting means to convergewith each other at the screen.

Although the invention has been described as applied to a color cathoderay tube having a single common cathode, and three separate first grids,it is to be understood that the present invention can be equally appliedto a color cathode ray tube including three separate cathodes and asingle first grid formed with three apertures through which the threebeams can pass respectively. Furthermore, it will be appreciated thatthe invention also can be applied to any color cathode ray tube in whichthe three beams originate in a horizontal plane and enter into the fieldproduced by horizontal-vertical deflection yoke means at predeterminedincident angles to each other, but without crossing each other at theoptical center of a main focussing lens, as in FIG. 1.

In the described embodiment of the invention, the three beams originatein a horizontal plane. However, these three beams may originate in avertical plane, in which case the horizontal deflection coils should bewound in a saddle-like form to produce a barrel shaped horizontaldeflection field, and the vertical deflection coils should be wound in atoroidal form to produce a pincushion shaped field. Furthermore, in sucha case, the

in place of the grid shown on FIG. 1 in opposing relationship to thescreen.

Having described a particular embodiment of the invention with referenceto the accompanying drawings, it will be understood that the inventionis notli'mited to such precise embodiment, and that various changes andmodifications, only some of which have been mentioned above, may be madetherein without departing from the scope or spirit of the invention.

What is claimed is:

1. In a color cathode ray tube having means generating plural beamsincluding a central beam and opposite side beams which originate in acommon plane and which are directed, at predetermined incident angles toeach other for convergence on a screen, through horizontal and verticaldeflection fields produced by electromagnetic deflection means and bywhich said beams are made to scan said screen, the improvementcomprising magnetic yoke means disposed wholly within said tube adjacentthe paths of said beams through said deflection fields and beingoperative to collect leakage flux from one of said fields and toconcentrate the collected leakage flux in a correction field throughwhich only said central beam passes for correcting deviations betweenthe rasters of said plural beams on said screen.

2. A color cathode ray tube according to claim 1, in which said magneticyoke means includes two spaced apart yoke members disposed at oppositesides of the path of said central beam and being shaped to collect saidleakage flux for concentration in the space between said yoke members.

3. A color cathode ray tube according to claim 2, in which each of saidyoke members includes a substantially straight portion extendingsubstantially parallel to and in spaced relation to the straight portionof the other yoke member, and end portions at the ends of said straightportion and being directed at substantial angles to the latter in thedirection away from said space between the yoke members.

4. A color cathode ray tube according to claim 3, in which said commonplane is horizontal and said straight portions of the yoke membersextend substantially vertically so that said yoke members collect andconcentrate said leakage flux from said vertical deflection field.

5. In a single-gun, plural-beam color picture tube which includes acolor screen having arrays of color phosphors and beam selecting meansprovided with apertures corresponding to said arrays, beam generatingmeans for directing a central electron beam and two side electron beamsin a common plane toward said screen for impingement on respectivephosphors of each array through the corresponding aperture, lens meansfor focussing said electron beams on said screen and having an opticalcenter at which said. beams are made to cross each other with said sidebeams emerging from said lens means along paths lying in said plane andwhich are divergent with respect to the central beam, electron beamconvergence deflecting means operative, upon the application of aconvergence deflecting voltage thereto, to deflect said side beamsemerging along said divergent paths for convergence of all of said beamsat an aperture of said beam selecting means, and deflection yoke meanshaving sweep signals applied thereto to provide fields which deflectsaid beams in directions respectively parallel, and at right angles tosaid plane for causing said beams to scan said screen; the improvementcomprising magnetic yoke means disposed wholly within said tube andcollecting leakage flux from said field which deflects said beams insaid direction at right angles to said plane and concentrating thecollected leakage flux in a correction field through which said centralbeam passes for correcting deviations between the positions of therasters on the color screen produced by said beams in scanning saidscreen.

6. A single-gun, plural-beam color picture tube according to claim 5, inwhich said magnetic yoke means includes spaced apart yoke membersarranged at opposite sides of said central beam and each having asubstantially straight portion at right angles to said plane betweensaid central beam and one of said side beams and end portions at theends of said straight portion and being directed at substantial anglesto the latter in the direction away from the other yoke member so thatsaid end portions act as pole pieces to collect said leakage flux forconcentration in said correction field established between said straightportions of the yoke members.

7; A single-gun, plural-beam color picture tube according to claim 6, inwhich said yoke members are mounted adjacent the entry end of saidconvergence deflecting means.

References Cited UNITED STATES PATENTS 2,925,542 2/1960 Gethmann.3,307,067 2/1967 Jachim et al 3'l5l3X 3,430,099 2/ 1969 Ashley.3,448,316 6/1969 Yoshida et al.

RODNEY D. BENNETT, Primary Examiner MALCOLM F. HUBLER, AssistantExaminer

